There is a growing number of photovoltaic (PV) applications where direct current/alternating current (DC/AC) inverters are required to provide independent maximum power point tracking (MPPT) at multiple inputs to harvest maximum solar energy from PV panels and to thereby feed clean AC electricity into the power grid. FIG. 1 is a circuit diagram of such an inverter using conventional technology and according to the prior art. As can be seen, each one of multiple power generators is independently coupled to a power conditioning sub-system with a DC/DC converter. Each sub-system has a power generator, a DC/DC converter, an energy storage module, a DC/AC inverter, and a specific digital controller that controls the various components of that sub-system. Unfortunately, this configuration is not only expensive but also quite complex, large, and heavy. Installation and maintenance for such systems are both costly and quite inconvenient.
Another issue which must be addressed is that, for each sub-system, the DC/AC inverter usually operates under hard-switching where neither the voltage nor the current of the power switches is zero during the switching transitions. The power semiconductors of the DC/AC inverter are switched under very high voltage and the intermediate DC-bus semiconductors in such inverters significantly contribute to the overall losses of the power conditioning system. In particular, reverse recovery losses of the power semiconductors' body diodes are inevitable for such topologies. Because of this, the switching frequency of the inverter is very limited, usually in the range of 10-20 kHz. Because of strict regulatory standards, high quality current needs to be injected into the utility grid from such inverter systems. To produce such high quality currents, such inverters require large filters at their outputs.
Another issue with low switching frequencies is that such low frequencies create a high amount of current ripple across the inverter output inductor. This current ripple not only increases the core losses of the inductor but also increases the inductor's high frequency copper losses. In addition to this, reducing the DC-bus voltage creates a significant amount of conduction and emission EMI noise. The high amount of conduction and emission EMI noise may affect the operation of the control system and may highly degrade the system's reliability. From all of the above, hard-switching limits the switching frequency of the inverter and imposes a substantial compromise in the design of the output filter and in the overall performance of the power conditioning sub-systems.
FIG. 2 shows a conventional three-phase DC/AC inverter according to the prior art. According to FIG. 2, the inverter includes three legs with each leg including two power semiconductors coupled in series. Each leg is coupled to an output inductor at a coupling node between the two power semiconductors. Each output inductor is then coupled to the power grid. The inductors filter out the switching harmonics of the inverter and produce a fairly clean sinusoidal current waveform. Due to the limited switching frequency of the DC/AC inverter, the output inductors are usually large and bulky in order to produce a reasonably clean current for the utility grid. While it is possible to use an LCL-filter instead of an L-filter at the output of the three-phase DC/AC inverter, six inductors would be required as well as sensing circuitry for the currents of the 6 inductors. Such an implementation of an LCL-filter for the output of the inverter would not be very cost-effective or practical.
Based on the above, there is therefore a need for systems and devices which mitigate if not avoid the shortcomings of the prior art.